There are two systems in spread spectrum modulation, direct sequence and frequency hopping (respectively, SS, DS and FH, hereinafter). Both systems have good characteristics not only of communication secrecy but also strength against jamming, narrow band interference, and multipath fading. Recently, in the field of wireless transmission, the CDMA (Code Division Multiple Access) mobile communication system using SS modulation has gained wide notice.
As radio waves are transmitted by dispersing transmission electricity to lower electric density in the SS system, high-speed acquisition is important for the object signal when receiving an SS signal. Acquisition means adjusting the timing of the receiving system to that of the transmitted signals. There are two types of acquisition, "obtaining acquisition" (acquisition, hereinafter), the state of unobtained acquisition at the initial communication, and "tracking acquisition" (tracking hereinafter), that watches acquisition so as not to lose it by modulation and noise.
DS acquisition means the work from presuming the timings of spreading code generation of transmitted signals and of its preparation in the receiver with the accuracy, for example, within one chip, to starting the action of spread spectrum generator of the receiver at the same timings.
The methods for performing acquisition are i) using a sliding correlator, and ii) using a matched filler.
In method i), spreading codes are generated in the receiver at roughly estimated timing,. and acquisition is attempted by gradual phase shifting of spreading codes. The received signals and spreading codes generated in the receiver are accumulated and passed through a low-pass filter. When the phase of spreading code sequence including the received signals coincides with that being generated in the receiver, a large amplification signal is obtained at the output of the low-pass filter. When the phases do not coincide, the signal is low-level as given by auto-correlation function of the spreading code sequence. Therefore, when the low-pass filter output is equal to or below the predetermined level, the phase of spreading code sequence generated by the spreading code generator is a little advanced or delayed. The spreading code sequence phase of received signals and that generated in the receiver coincide by repeating the above processing.
By this method, the phases can be made to coincide by shifting the phase of the spreading code generator in most instances corresponding to one cycle of spreading code sequence. When the spreading code generator works in a given phase, one cycle's span of spreading code sequence is necessary to investigate whether that phase is adequate or not. Therefore, this method requires one cycle's spreading code sequence time X the spreading code sequence length to complete the acquisition.
Method ii) soon detects correlation value, allowing acquisition to be completed at once. When SS modulation signals are successively input from the receiving side of a matched filter, the phase value at every moment appears at the output. Therefore, the peak of correlation can be detected by observing the outputs of the matched filter during the time of one cycle of spreading codes. With this method, the acquisition can be completed in the time corresponding to one cycle of spreading codes.
When the acquisition is completed, watching and correcting it are necessary so as not to lose its location caused by the influence of modulation and noise. This is tracking, for which a DLL (Delay Locked Loop) circuit is usually used. This tracking circuit includes two sets of correlators: each correlator de-spreads the received signals with different phase shifted sequence of the same spreading code, for example, that is a half-chip advanced and half-chip delayed than those used in active de-spreading, and the difference of outputs of the correlators is calculated after passing through envelope detecting circuit. An S-shaped correlation output can be obtained by it. Acquisition is performed by controlling through feeding back the phase of spreading codes generated in the receiver using the S-shaped correlation output.
In mobile radio communication, multipath fading is generated because the signals transmitted from the base station reach a receiver through a plurality of transmission routes with different route lengths of which are not added coherently. As an effective countermeasure against such multipath fading, the RAKE receiving method using a direct spread spectrum signal realizes path-diversity by distinguishing the signals taking multiple paths, adding weight of reliance, and combining.